The C3b receptor from human E has recently been purified to homogeneity. Monospecific rabbit antiserum to this glycoprotein of 205,000 molecular weight is able to immunoprecipitate like molecules from human neutrophils, monocytes and B lymphocytes. The antiserum is able to block C3b receptor-dependent rosettes by peripheral blood cells, and to bind specifically only to those cells expressing the C3b receptor. The objectives of this proposal are to produce mouse monoclonal antibodies to the human C3b receptor and the analyze these monoclonal antibodies for their capacity to cross-react with C3b receptors on cells from other species, to distinguish between C3b receptors on different human cell types or to recognize allotypic determinants of the receptor, and to distinguish between receptor sites that are unoccupied or occupied by C3b ligand.